The present invention relates generally to automotive dampers or shock absorbers which receive mechanical shock. More particularly, the present invention relates to a unique hydraulic valve assembly which allows greater tunability of the shock absorber, especially in the mode of low speed or low hydraulic fluid flow.
Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb these unwanted vibrations, shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (wheels) of the automobile. A piston is located within a working chamber defined by a pressure tube of the shock absorber, with the piston being connected to the sprung portion of the automobile through a piston rod. The pressure tube is connected to the unsprung portion of the vehicle by one of the methods known in the art. Because the piston is able, through valving, to limit the flow of damping fluid between opposite sides of the piston when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which damps the unwanted vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile. In a dual tube shock absorber, a fluid reservoir is defined between the pressure tube and the reserve tube. A base valve can be located between the lower portion of the working chamber (the area below the piston) and the reservoir to limit the flow of fluid between the lower working chamber and the reservoir. When both piston valving and a base valve are utilized, the piston valving produces a damping force which counteracts the unwanted vibrations during an extension stroke of the shock absorber and the base valve produces a damping force which counteracts the unwanted vibrations during a compression stroke of the shock absorber. The greater degree to which the flow of fluid within the shock absorber is restricted by the piston valving and the base valve, the greater the damping forces which are generated by the shock absorber. Thus, a highly restricted flow of fluid would produce a firm ride while a less restrictive flow of fluid would produce a soft ride.
In selecting the amount of damping that a shock absorber is to provide, at least three vehicle performance characteristics are considered. These three characteristics are ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant for the main springs of the vehicle as well as the spring constant for the seat and tire and the damping coefficient of the shock absorber. For optimum ride comfort, a relatively low damping force or a soft ride is preferred.
Vehicle handling is related to the variation in the vehicle""s attitude (i.e., roll, pitch and yaw). For optimum vehicle handling, relatively large damping forces, or a firm ride, are required to avoid excessively rapid variations in the vehicle""s attitude during cornering, acceleration and deceleration.
Finally, road handling is generally a function of the amount of contact between the tires and the ground. To optimize road handling ability, large damping forces, or a firm ride, are required when driving on irregular surfaces to prevent loss of contact between the wheel and the ground for excessive periods of time.
Various types of shock absorbers have been developed to generate the desired damping forces in relation to the various vehicle performance characteristics. Shock absorbers have been developed to provide different damping characteristics depending upon the speed or acceleration of the piston within the pressure tube. Because of the exponential relation between pressure drop and flow rate, it is a difficult task to obtain a damping force at relatively low piston velocities, particularly at velocities near zero. Low speed damping force is important to vehicle handling since most vehicle handling events are controlled by low speed vehicle body velocities.
Various prior art systems for tuning shock absorbers during low speed movement of the piston create a fixed low speed bleed orifice which provides a bleed passage which is always open across the piston. This bleed orifice can be created by utilizing orifice notches positioned either on the flexible disc adjacent to the sealing land or by utilizing orifice notches directly in the sealing land itself. The limitations of these designs is that because the orifice is constant in cross-sectional area, the created damping force is not a function of the internal pressures of the shock absorber. In order to obtain the low speed control utilizing these open orifice notches, the orifice notches have to be small enough to create a restriction at relatively low velocities. When this is accomplished, the low speed fluid circuit of the valving system will operate over a very small range in velocity. Therefore, the secondary or high-speed stage valving is activated at a lower velocity than is desired. Activation of the secondary valving at relatively low velocities creates harshness because the shape of the fixed orifice bleed circuit force velocity characteristic is totally different than the shape of the high speed circuit.
Prior art attempts at overcoming the problems of fixed orifice bleed valving and thus eliminate harshness during low speed piston movements have included the incorporation of a variable orifice bleed valving circuit. As the velocity of the piston increases, the flow area of the variable orifice also increases to smooth the transition to the secondary valving. These prior art variable orifice bleed valving circuits are typically located at the outer periphery of the flexible valve disc and thus they are dependent on the diameter of the disc to determine the rate at which the flow area increases. As the diameter of the flexible disc increases, it becomes more difficult to control the rate at which the flow area of the orifice increases. Since the flow area is increased by the deflection of the variable orifice bleed disc, a small deflection in a large diameter variable orifice bleed disc provides a rapid increase in the flow area of the bleed orifice. This rapid increase in the flow area complicates the tuning between the low speed valving circuit and the secondary or high-speed valving circuit.
Still other prior art systems have developed variable bleed valving circuits which are integrated with the mid/high-speed valving systems. The integration of the low speed circuit with the mid/high speed circuit creates a system where the tuning of the low speed circuit affects the mid/high-speed circuit and the tuning of the mid/high-speed circuit affects the low speed circuit.
The continued development of shock absorbers includes the development of a valving system which can provide a smooth transition between a low speed valving circuit and the secondary or high speed valving circuit. The smooth transition between these two circuits helps to reduce and/or eliminate any harshness during transition. In addition to the smooth transition, the development of these systems has also been directed towards the separation of these two circuits in order to be able to independently tune each of these circuits.
The present invention provides the art with a method for independently tuning damping forces at low piston velocities in order to improve the handling characteristics of the vehicle without creating harshness. The present invention provides a low speed variable orifice bleed circuit which is separate from the mid/high-speed circuit of the secondary valving system. The secondary valving system of the present invention includes a first disc secured to the piston to close the mid/high-speed extension passages extending through the piston. The first disc deflects due to a pressure differential to open the mid/high-speed extension fluid passages during the second storage valving. The low speed variable orifice bleed circuit of the present invention includes a plurality of discs secured to the piston but separate from the first disc. The second plurality of discs close the low speed extension fluid passages extending through the piston. The second plurality of discs also deflect due to a pressure differential to open the low speed extension fluid passages during the initial stage valving. The separation of these two valving systems allows the designer to separately optimize the tuning of each valving system to optimize the tuning of each varying system to optimize the damping forces created by the shock absorber during an extension stroke and thus improve the vehicle handling without creating harshness. A similar dual valving system can be incorporated into the base valve of the present invention to optimize the damping forces created during a compression stroke.
Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.